746
FXUS63 KLOT 121952
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
252 PM CDT Wed Jun 12 2024

.KEY MESSAGES...

- Weakening showers/storms may move into the northern portion of
  the CWA late tonight or early tomorrow morning.

- Level 2 to 3 out 5 risk for severe weather Thursday afternoon/evening
  for large hail and strong winds. There is also a localized
  flooding threat.

- Hot and humid conditions are expected early next week, though
  thunderstorms may occur at times and modulate the heat.

&&

.DISCUSSION...
Issued at 252 PM CDT Wed Jun 12 2024

Through Thursday Night:

It will continue to be hot (but with thankfully tolerable
dew points/humidity levels) through the remainder of the day
with temperatures holding in the upper 80s to lower 90s until
near sunset. The lake breeze hugging the Illinois shore will get
pushed back offshore and result in rising temps there into this
evening.

Zonal northwest flow across the northern Plains has
already allowed a cluster of storms to get going. As these
storms potentially approach the far northwestern and far
northern CWA this evening, they will be in a weakening phase
with isolated showers and perhaps an embedded rumble of thunder.
Another round of storms will likely form later tonight and
advance southeast through MN and WI towards the northwestern and
far northern CWA. Dry low to mid-level air and a capping
inversion across the area will result in these storms weakening
as they reach the CWA.

Although unlikely, there is a chance that a few of these storms
both tonight and into early tomorrow morning could pose a
strong to locally severe wind threat if outflow is able to
maintain itself (best chance far NW CWA). Outflow may maintain
long enough for a breezy/variable wind shift as far southeast as
the Chicago metro around dawn. Kept PoPs in the 15-35% range
through the late morning with the best chance for showers and
storms around sunrise Thursday morning across the northwest 1/3
to 1/4 of the CWA.

Thursday afternoon, models are in good agreement that southwesterly
flow will allow moisture to build into the area with dew points in
the upper 60s (more aggressive models reach the low 70s). With
temperatures rising to around 90 degrees, MLCAPE values in the
2000-3000 J/kg range are expected. A surface cold front will be
moving southeast towards the CWA Thursday afternoon. Initially
weak surface convergence, a cap, and weak upper-level support
will inhibit storms early in the afternoon, however, as frontal
convergence increases and a modestly strengthened mid-level jet
moves overhead, storms will become more likely through the late
afternoon.

The large CAPE values and 35-45 kts of shear will cause storms
to become supercellular in nature early on. Steep mid-level
lapse rates (7-8 degrees C/km) will present a threat for large
to locally destructive (2"+ diameter) hail with large DCAPE
values (>1000 J/kg) bringing a threat for strong to severe
winds (70-80 mph gusts possible). These storms will also pose a
localized flooding threat with heavy rainfall rates and PWATs
forecasted to be around or potentially over 2 inches. The threat
for severe weather has warranted a level 3 out of 5 risk for
the southwestern portion of the CWA (where confidence in higher
coverage of severe weather is highest) and a level 2 out of 5
risk for the remainder of the CWA from SPC.

Depending how the convection plays out Thursday morning could impact
how things unfold Thursday afternoon/evening. If showers/storms hold
on through the late morning across the area, then temperatures will
take longer to rebound and instability will struggle to build early
in the afternoon. This stability would limit storm initiation
chances until later Thursday allowing the cold front to advance
further south and thus moving the highest threat for storms and
severe weather further south as well.

However, if showers dissipate before they reach the CWA, then
temperatures and instability will be able to build quickly and
storms may be able to form further north during the afternoon.
The 12Z HRRR seems to be playing more into this scenario, which
would be a "worst case" scenario for this setup, featuring
convective initiation and the anticipated level 3 type threats
currently farther southwest to likely include more of the
Chicago metro. This will be monitored closely.

Initial supercell development will likely grow upscale into clusters
or a more linear mode as the evening progresses. Once the convection
moves out of the CWA Thursday night, things will quiet down
into Friday morning.

Carothers/Castro

Friday through Wednesday:

Rest of discussion to follow shortly.

NWS Chicago

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1249 PM CDT Wed Jun 12 2024

The primary aviation weather concerns through the 18Z TAF period
include:

* A potential for a few gusty showers early Thursday morning

* Showers and thunderstorms Thursday evening

SW winds near or under 10 kt will continue through the day.
Overnight, sustained winds are expected to be predominantly less
than 10 kt. However, occasional gusts will be possible throughout
the night.

A decaying system will likely bring showers to RFD late tonight into
early Thursday. There remains a good deal of uncertainty regarding
whether the showers make it to the Chicago metro early Thursday
morning. These showers will have the potential to produce gusty
outflow winds which could cause wind direction to get rather erratic
while they remain nearby.

Breezy SW winds can be expected for the better part of Thursday.
Late in the afternoon, a system of showers and thunderstorms will
approach from the north. Confidence is low on coverage and timing,
but most model guidance suggests we`ll have at least pockets of
precip in the metro by 22Z. Similarly to the morning showers, these
storms could pack a good punch of outflow wind which could result in
sporadic wind shifts. This potential will continue beyond the
current 30-hr TAF period.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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